In a large pot of salted boiling water, cook the potatoes for about 20 minutes or until tender. Drain.
With a potato masher, crush the potatoes with the butter. Add the cream and purée until smooth. Season with salt. Set aside.
With the rack in the middle position, preheat the oven to 350°F (180°C).
In a large non-stick skillet over high heat, brown half the fish cubes at a time in 2 tbsp of the butter. Season with salt and pepper. Set aside on a plate. Discard the cooking liquid.
In the same skillet, soften the vegetables in the remaining butter (2 tbsp) for 5 minutes. Season with salt and pepper. Deglaze with the wine. Add the broth and cream. Let simmer over medium-high heat for about 10 minutes or until the sauce has reduced by three-quarters. Drain the fish pieces and add to the sauce.
Spoon the fish mixture into four 1 ½-cup (375 ml) ramekins or into an 8-inch (20 cm) square baking dish. Spread the mashed potatoes over the fish. (At this point, the shepherd’s pie can be refrigerated or frozen.)
Bake for 20 minutes (or 40 minutes if refrigerated or frozen). Finish under the broiler, if desired. Let cool for 10 minutes before serving.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.